1. Field of the Invention
The present invention relates generally to non-destructive testing (NDT) of pipes, tubes, and other cylindrical shell structures for flaws and fractures. The present invention relates more specifically to a non-destructive evaluation (NDE) system and method for flaw detection in pipes, tubes, and other cylindrical shell structures using magnetostrictively generated mechanical waves and magnetostrictively detected mechanical waves reflected from flaws.
2. Description of the Related Art
Pipes, tubes, and other cylindrical shell structures are important components in a variety of industries, including the power generating, pipeline, and petrochemical industries. The failure of pipelines and other such conduits associated with these industries can often be disastrous, leading to the loss of operating revenue, extensive structural damage, environmental contamination, unscheduled plant shut-downs, and in extreme cases even the loss of life. The inspection of such pipes, tubes, and conduits is, therefore, an essential part of system maintenance procedures in each of these industries.
At present, various inspection techniques that include ultrasonic, eddy current, x-ray, and magnetic flux leakage methods, are used for such pipeline, tube, and conduit inspections. In general, these techniques are well suited for localized inspections, as for example, the inspection of individual welded areas. However, for situations where defects can occur at any place in a long pipe or tube (for example, in pipes under thermal insulation or wrapping in petrochemical processing plants), or where the number of pipes to be inspected is large (for example, in steam generators and boilers), inspection using any of these techniques can be very time consuming and expensive. In such cases, inspections are very often performed only on a select portion of the pipes as a sample intended to be indicative of the overall condition of the entire group. This may or may not be satisfactory for assuring system safety and reliability and merely reduces the risk of failure a small degree.
As an example of current methods, gas pipelines, which generally extend over hundreds of miles, are sometimes inspected for corrosion using a magnetic flux leakage "pig". Such methods are at best incomplete since the detection of cracks in gas pipelines using the magnetic flux leakage method is not particularly sensitive. Ultrasonic techniques, which might be sensitive enough to detect smaller cracks, are often difficult to apply to pipelines because of the transducer couplant requirements.
Magnetostrictive sensor technologies remove many of the above described disadvantages of conventional NDE techniques and can significantly improve the efficiency of inspection. The salient features of the present invention that derive from the use of a magnetostrictive sensor system include the ability to; inspect 100% of the pipe cross-section, inspect long segments of pipe (more than 100 feet) from a single location, detect defects on both the outside and inside diameter surfaces of the pipe, function without need for a couplant, and operate at a low cost of inspection.
Magnetostrictive sensors may be applied either from the outside diameter of the pipe or, in appropriate situations, on the inside diameter of the pipe. Normally, magnetostrictive sensors are most easily applied to pipes, tubes, and other cylindrical shell structures that are made of ferromagnetic materials. However, for many non-ferrous pipes, tubes, etc. (Inconel steam generator tubes for example), magnetostrictive sensors can still be utilized by plating or bonding thin layers of magnetizable materials, such as nickel, on the pipe or tube in a localized area where the sensor is to be placed. In this way, the methods involved in magnetostrictive wave generation and detection can be utilized in association with both ferrous and non-ferrous pipes, tubes, and cylindrical shell structures.
Applicant's co-pending patent applications Ser. Nos. 07/973,152 and 08/033,256 describe in greater detail various methods for utilizing magnetostrictively generated waves and magnetostrictively detected waves in long, solid, cylindrical structures such as cables and ropes. Hollow cylindrical structures such as pipes, tubes, etc., present specific geometric and wave propagation characteristics that require some modification of the methods and devices associated with magnetostrictive non-destructive evaluation. Heretofore, little effort has been made to develop such methods and devices appropriate for the magnetostrictive analysis of hollow cylindrical shell structures.